We conducted a “megastudy” with 3.5 million pharmacy patients in November 2022 simultaneously testing eight behaviorally-informed interventions aimed at encouraging Covid-19 booster vaccinations in the United States. Many eligible Americans have not yet received a booster, suggesting a need for new ways of effectively encouraging vaccination to counter inaccurate health information, low motivation, and logistical barriers. Our megastudy testing new means of encouraging booster vaccination was designed by an interdisciplinary team of scientists. We found that sending two sets of vaccination reminders seven days apart resulted in a 20% relative increase in 30-day Covid-19 booster vaccinations and generated a positive spillover effect, increasing 30-day flu vaccinations by 7%. Offering patients free round-trip Lyft rides to pharmacies had no benefit over and above sending simple reminders. Both experts and laypeople incorrectly predicted that offering free rides would outperform sending reminders. In Fall 2023, we are conducting a follow-up, large scale RCT with over 1 million pharmacy patients to further investigate our megastudy’s top-performing intervention.